Not applicable
1. Field of the Invention
The present invention relates to a screen assembly for applying decorative imprints to articles, and more particularly, the present invention relates to the construction of carrier for a silk screen including at least one but preferably both of upstanding elongated sidewalls and screen frame support arms each having sufficient resiliency to restore the screen to a screen support plane after a squeegee displaced the screen when forcing a printing medium through a desired pattern in the screen.
2. Description of the Prior Art
Silk screen printing is well known in the art and takes it name from the original use of a screen made of woven silk thread. More recently, the silk screen printing process uses a screen made from thread-like filaments comprised of materials such as plastics e.g., rayon, when printing with solvent based paint and metal e.g. stainless steel to allow electrically resistive heating of the screen when printing with thermo plastic paint. Silk screen printing is extensively employed to apply decoration to surfaces of mass-produced articles of manufacture. The decoration may be applied to produce simple schemes or patterns or they may be applied in more elaborate decorative designs including manufacturer""s names, product names, logos, and similar types of information. During printing, the paint medium such as ink is forced by a squeegee through exposed areas of the screen, which delineate the intended design to be applied to the surface of the article. Examples of prior art silk screen printing apparatus using thermo plastic paint are disclosed in U.S. Pat. Nos. 3,894,487; 4,137,842; and 4,520,727. Thermo plastic paint is maintained in a fluid state by electric resistive heating of a metallic wire mesh screen secured to but electrically insulated from a metal support frame. The wire mesh screen is stretched between edges of the frame so that end portions of the wire mesh can be wrapped to extend along the sides of the frame and secured thereto. The screen is usually stretched to maintain sufficient tension to eliminate wrinkles in the screen and prevent distortions of the desired pattern usually located in a central area of the screen surrounded by the sides of the frame. The screen is stretched by the squeegee to a small extent each time the printing medium is forced through the paint-pervious openings by the squeegee. Sometimes the frame is moved along a path of travel relative to a stationary squeegee to force ink through the paint-pervious openings in the screen; however, other drive systems for the printing process provide that the squeegee move along a path of travel relative to a stationary screen. 
After prolonged use of the screen, the metal or other material used to form the screen fatigues producing distortion to the intended design and leading to the screen failure usually appearing as a tear in the screen. When this occurs, the screen must be replaced and usually the entire screen assembly is removed from the decorating machine and a replacement screen installed on the machine. The rigorous use of the screen assembly particularly in a high through put volume decorating machine required a robust frame construction to withstand the repetitive flexing of the screen by the squeegee. The use of a heavy iron casting with thick wall construction is typical. The cost of the cast iron frame adds to the printing costs in terms of the initial investment and maintenance needed to clean encrusted printing medium from the frame before each periodic replacement of the silk screen. The foregoing patents addressed extending the fatigue life of the screen material by attaching a resilient gasket to the lower edge surface of the frame to avoid stretching of the screen across the sharp edges of the cast iron frame. However, fatiguing of the screen material still occurs because the screen must be pressed into contact with the workpiece by movement of the squeegee during movement relative to the screen. The screen is normally positioned a short distance from the surface of the workpiece so that an air gap exists between the screen and the workpiece. The squeegee is moved to displace the screen by the distance of the air gap into contact with the workpiece.
In the printing operation, the silk screen is pushed slightly outwardly in a downward direction from the frame and drawn taut by the squeegee during each design transfer. Upon completion of the transfer of the design, the squeegee is retracted and the screen is permitted to relax to its original condition. The screen is adaptable to printing on a wide variety of surfaces and/or articles of manufacture, but unfortunately, particularly when printing on a flat substrate, the continuous tensioning and relaxing of the wire mesh material of the screen soon causes unavoidable and harmful fatigue of the material, particularly where the material is in contact with the lower edges of the metallic frame, hence leading to failure of the screen. A further disadvantage of silk screen apparatus of this type is that the wire mesh screen cannot always be tensioned uniformly over the entire area encircled by the screen support frame. For example, the screen is usually tensioned and adhered by cement along substantial portions of the frame beyond the rubber boot which surrounds the bottom edge of frame while the remaining portions of the screen are held in metal compression clamps, which act as electrical bus bars at times when the screen is electrically heated. The compression clamps are usually adjustable by screw fasteners to opposite ends of the screen support frame.
Due to its inherent lack of elasticity, a metallic wire mesh screen is difficult to stretch across the rubber boot under sufficient tension to cause the screen to tightly conform to the shape of the frame. Consequently, the wire mesh screen can only be effectively tensioned along its length dimension where its clamped ends are adjustably connected to the frame by the aforementioned screw fasteners. Such non-uniform tensioning of the screen has been known to produce wrinkles and/or other variably tensioned areas in the screen which deleteriously affect the quality of the image which is transferred by the screen to the article. Such screens further have relatively limited inherent resiliency. This generally does not create a problem during many printing operations since the resiliency of the screen was used to accommodate the tensioning and relaxing by the squeegee. However, at times when thermally responsive printing medium is applied by the screen printing apparatus, i.e., at times when the screen and printing medium are heated, the thermally responsive printing medium, if applied to a substantially cooler surface such as glass, for example, quickly sets upon contact therewith. In such thermally dependent printing operations, it would be most advantageous if the screen structure could be provided with greater inherent resiliency whereby it would positively and reliably return to its initial position upon passage of the squeegee to thereby prevent sticking or blocking of the screen to the cooler article surface. U.S. Pat. No. 4,373,441 and German Offenlegungsschrift No. 29 16 391 disclose composite silk screen apparatus which require elaborate constructions involving electroplating opposite end portions of a metallic silk screen that are thermo plastically secured to a thermoplastic synthetic resin material such that the thermoplastic securement between the wire silk screen and the resin material is not damaged during electrical resistance heating of the silk screen.
An advantage exists, therefore, for a silk screen assembly where a metal frame to support the silk screen is constructed with sufficient resiliency to impart a resistance to metal fatigue of the silk screen and maintain a uniform resiliency across the screen during a printing operation.
It is accordingly an object of the present invention to provide a frame assembly for supporting a silk screen to resist metal fatigue and allow flexing of the frame assembly when the silk screen is displaced by a squeegee to force printing medium through the openings in the silk screen.
It is a further object of the present invention to provide a silk screen printing apparatus having screen frame assembly to tension a printing screen in all directions within the plane of the silk screen to thereby provide a high-quality image transfer surface.
It is a further object of the present invention to provide a silk screen printing assembly having a screen frame construction designed to exhibit a useful highly resilient property and, therefore, especially useful in thermally dependent printing applications wherein a substantial temperature differential exists between the silk screen and the article surface being coated such a highly resilient printing screen frame construction thereby preventing adhering of the screen to the article surface which is caused by the rapid cooling and setting of the thermally responsive printing medium upon contract with the cooler article surface.
It is another object of the present invention to provide a silk screen printing apparatus having a screen frame construction, which permits accurately controllable and essentially uniform electrical resistance heating of the screen in the instances where the printing medium is thermally responsive.
It is an another object of the present invention to provide a screen frame construction having resilient longitudinal side walls and resilient screen support arms for attaching a frame for support so that the resiliency of the side walls and/or support arms can withstand repeated flexing movement of the middle portion of the screen toward and away from the workpiece as a printing medium is forced through a desired pattern in the screen.
Thus, in accordance with the present invention the foregoing problem of fatiguing of the screen material is reduced by providing a stencil screen assembly including the combination of a stencil screen having a desired pattern to be imprinted defined by an adhered paint-impervious layer, the stencil screen having sufficient strength transversely to plane of the screen to allow deflection from the plane of the screen without elastic deformation, a rectangular stencil screen frame including upstanding elongated sidewalls joined by spaced apart upstanding end walls to circumscribe a screen window opening establishing a stencil screen support plane, and screen support arms secured to the rectangular stencil screen frame to extend outwardly from the end walls, at least one of the upstanding elongated sidewalls and the screen support arms having sufficient resiliency to allow displacement and restoration of the stencil screen to the stencil screen support plane when displaced there from by a squeegee while traversing the desired pattern to be imprinted.
Preferably, the upstanding elongated sidewalls having sufficient resiliency to restore the stencil screen to the stencil screen support plane when displaced there from by the squeegee. The upstanding elongated sidewalls and upstanding end walls are defined by L shaped cross sectional configurations having upstanding heights defining an ink reservoir volume above an internally projecting foot mitered at juncture sites between each of the side walls and the end walls, the projecting foot of each of the upstanding elongated sidewalls defining a screen mounting shelf. An electrically insulating coating on the upstanding elongated sidewalls and the upstanding end walls electrically isolates the stencil screen frame from the stencil screen and an electrically insulating layer of adhesive bounds an outer marginal edge portion of the stencil screen to the mounting shelf of the sidewalls. The stencil screen includes electrically conductive terminal end portions extending from the upstanding elongated sidewalls along the screen support arms and pressed into electrically conductive contact with underlying bus bars substantially corresponding to the width of the stencil screen.